Transcript
This presentation is about Intrafraction patient movement in spinal IMRT, and the consequences of this movement for the CTV-PTV margins. We did this work with our colleagues of the Clinical Physics department, Jaap van Egmond, and Jan van Santvoort. The kind of patients we talk about are patients with a target in the spinal or the paraspinal areas, and we want to give them a dose that is, a CTV a dose that is higher than the tolerance dose of the spinal cord. And we usually do that with IMRT with Novalis. These treatments have a duration of at least 20 to 30 minutes. So, in this time, patients can move and we should take that into account.
The purpose of this study was to quantify the inter- and intrafraction movement of the patients and to establish CTV-PTV margins based on these movements. This is what we did. We treated 16 patients with spinal and paraspinal targets, and we all gave them fractionated treatments. In total were 312 fractions. The setup was done using ExacTrac and errors were corrected. And after correction, the translation error was the first data we used.
Patients were treated, and after treatment twice a week we did a check of the position right after the treatment. And in the 16 patients, we did 124 checks after the fraction. And these data we used also.
These are some data about the 16 patients we treated. There were five patients with lung cancer. Four of the five had a fixation with a thermoplastic mask because they were all sulcus superior tumors, and one patient had a tumor that was lower that had no fixation. And we had nine patients with metastasis. Three metastasis were in the cervical region, and they were also treated in a thermoplastic mask.
This is how the setup is done in our department. The patients with cervical targets have just an ordinary, had a neck mask system on Novalis. And targets in other areas for those patients with...there was no fixation. Patients were just positioned as comfortably as possible. We used no vacuum cushions or something.
Now to the results. First, to give you an impression of the magnitude of the errors we've seen, we show you the vectors, the 3D vectors. After translations, we did not use the rotational errors. We just looked at the translational errors. Before treatment, the errors were very small as you can see, 0.8 millimeters with a small standard deviation. But also after treatment, the errors were remarkably small so patients managed to stay still without much movement during the whole treatment of 20 to 30 minutes.
There was an error of just 1.5 millimeters. It was a standard deviation that was a little bit larger. But to calculate CTC to PTV margins, we need systematic errors and random errors in all three directions. In this table, you see these errors? They were very very small in the pre-treatment situation, after correction. And here you see the errors after treatment, and the errors were a little bit larger but still they were very small. In fact, we were very surprised that these errors can be so small without any form of fixation. The use of the immobilization mask had no significant influence on these errors.
To calculate the CTV-PTV margin, we used the formula from Rotterdam, from [inaudible 00:04:24] in 1999. And the margin, we calculate by two times the systematic errors plus 0.7 times the random errors. When we calculated the margins, the CTV-PTV margin based just on the pre-treatment errors, you could think that you could use very small CTV-PTV margins. By the way, the formula for the margins are just for fractionated treatments.
And when you use also the pre- and the post-treatment errors, the margins should be a little bit larger, the CTV-PTV margins. But still, the margins are very small. And with these small margins, you should take into account other inaccuracies also because you have to take into account the inaccuracy of the CT-scan, and the slice thickness, the dimensions of the Isocenter, inaccuracy of the lasers, the Exactrac, the MLC, and the dose computation. And the final margin can be calculated using this formula, calculating the root of the sum of the squares of all these inaccuracies.
The largest inaccuracy of all, of course, is the work of the doctor when he is contouring, but we cannot take that into account. We don't know how to do it. And when all these inaccuracies are taken into account, we come to these margins in the three directions, 2.2, 2.7, and 2.8 millimeters. And we think it's practical to take 3 millimeter CTV-PTV margin for fractionated IMRT on spinal targets.
That was the result of the study and I want to discuss two points based on these results. First is the question, is the error that you measure on the end of the treatment, what does it say about the period of the treatment? Hoogeman from Rotterdam published results from patients treated with the CyberKnife that were...the setup was checked every two and a half minutes. And he found in the spinal patients, those other patients who are prone, that there was a linear increase of the inaccuracies. So, to our opinion, you can be very confident that the error you find on the end of the treatment is about the largest error during treatment and that during treatment the errors probably are not larger than that.
Furthermore, you can see that the errors in the CyberKnife situation are a little bit larger than what we found because the patient have to be prone with the CyberKnife.
Second point of discussion is what would be the clinical consequences of these findings. That's difficult to formulate. It's easy...in this kind of situation, patient with the paraspinal tumor that is on some distance from the spinal cord, then it's easy to do a CTV-PTV margin of 3 millimeters and still treat the spinal cord to a dose that is lower than the tolerance dose. But this kind of situation is more difficult.
This was a young breast cancer patient. She had early breast cancer five years before this MRI was made. And this metastasis in C1 was a solitary metastasis. So our opinion was that we had to go for a long-term local control and that the CTV, what you see here, is very close. It's in contact with the spinal cord. So what we decided to do was, first, surgery. There was a transoral resection of the dens so that the distance from the tumor to the spinal cord was larger. You can see that on this slide.
This is the microscopic rest of the tumor and this is spinal cord. There's now a distance after the surgery, so you can easily make now a CTV-PTV margin of 3 millimeter and treat this patient to a high dose with fractionated IMRT. But, of course, there are other situations where this kind of surgery is not possible and you can make the CTP to PTV margin of 3 millimeters there but you can, obviously, not do that here. This tumor was growing through the intervertebral foramen. And while it made contact with the spinal cord, it was not possible to make a CTV-PTV margin of 3 millimeter. And, well, you have to accept a lower dose in this region of the tumor, of course, if you don't want to overdose the spinal cord.
Concluding. Intrafraction movements are very small in most patients. CTV-PTV margins of 3 millimeters are safe in our view, and when the distance of the CTV to the spinal cord is less than 3 millimeter, either neurosurgery has to be considered or a lower dose in the CTV adjacent to the spinal cord has to be accepted. Thank you.
The purpose of this study was to quantify the inter- and intrafraction movement of the patients and to establish CTV-PTV margins based on these movements. This is what we did. We treated 16 patients with spinal and paraspinal targets, and we all gave them fractionated treatments. In total were 312 fractions. The setup was done using ExacTrac and errors were corrected. And after correction, the translation error was the first data we used.
Patients were treated, and after treatment twice a week we did a check of the position right after the treatment. And in the 16 patients, we did 124 checks after the fraction. And these data we used also.
These are some data about the 16 patients we treated. There were five patients with lung cancer. Four of the five had a fixation with a thermoplastic mask because they were all sulcus superior tumors, and one patient had a tumor that was lower that had no fixation. And we had nine patients with metastasis. Three metastasis were in the cervical region, and they were also treated in a thermoplastic mask.
This is how the setup is done in our department. The patients with cervical targets have just an ordinary, had a neck mask system on Novalis. And targets in other areas for those patients with...there was no fixation. Patients were just positioned as comfortably as possible. We used no vacuum cushions or something.
Now to the results. First, to give you an impression of the magnitude of the errors we've seen, we show you the vectors, the 3D vectors. After translations, we did not use the rotational errors. We just looked at the translational errors. Before treatment, the errors were very small as you can see, 0.8 millimeters with a small standard deviation. But also after treatment, the errors were remarkably small so patients managed to stay still without much movement during the whole treatment of 20 to 30 minutes.
There was an error of just 1.5 millimeters. It was a standard deviation that was a little bit larger. But to calculate CTC to PTV margins, we need systematic errors and random errors in all three directions. In this table, you see these errors? They were very very small in the pre-treatment situation, after correction. And here you see the errors after treatment, and the errors were a little bit larger but still they were very small. In fact, we were very surprised that these errors can be so small without any form of fixation. The use of the immobilization mask had no significant influence on these errors.
To calculate the CTV-PTV margin, we used the formula from Rotterdam, from [inaudible 00:04:24] in 1999. And the margin, we calculate by two times the systematic errors plus 0.7 times the random errors. When we calculated the margins, the CTV-PTV margin based just on the pre-treatment errors, you could think that you could use very small CTV-PTV margins. By the way, the formula for the margins are just for fractionated treatments.
And when you use also the pre- and the post-treatment errors, the margins should be a little bit larger, the CTV-PTV margins. But still, the margins are very small. And with these small margins, you should take into account other inaccuracies also because you have to take into account the inaccuracy of the CT-scan, and the slice thickness, the dimensions of the Isocenter, inaccuracy of the lasers, the Exactrac, the MLC, and the dose computation. And the final margin can be calculated using this formula, calculating the root of the sum of the squares of all these inaccuracies.
The largest inaccuracy of all, of course, is the work of the doctor when he is contouring, but we cannot take that into account. We don't know how to do it. And when all these inaccuracies are taken into account, we come to these margins in the three directions, 2.2, 2.7, and 2.8 millimeters. And we think it's practical to take 3 millimeter CTV-PTV margin for fractionated IMRT on spinal targets.
That was the result of the study and I want to discuss two points based on these results. First is the question, is the error that you measure on the end of the treatment, what does it say about the period of the treatment? Hoogeman from Rotterdam published results from patients treated with the CyberKnife that were...the setup was checked every two and a half minutes. And he found in the spinal patients, those other patients who are prone, that there was a linear increase of the inaccuracies. So, to our opinion, you can be very confident that the error you find on the end of the treatment is about the largest error during treatment and that during treatment the errors probably are not larger than that.
Furthermore, you can see that the errors in the CyberKnife situation are a little bit larger than what we found because the patient have to be prone with the CyberKnife.
Second point of discussion is what would be the clinical consequences of these findings. That's difficult to formulate. It's easy...in this kind of situation, patient with the paraspinal tumor that is on some distance from the spinal cord, then it's easy to do a CTV-PTV margin of 3 millimeters and still treat the spinal cord to a dose that is lower than the tolerance dose. But this kind of situation is more difficult.
This was a young breast cancer patient. She had early breast cancer five years before this MRI was made. And this metastasis in C1 was a solitary metastasis. So our opinion was that we had to go for a long-term local control and that the CTV, what you see here, is very close. It's in contact with the spinal cord. So what we decided to do was, first, surgery. There was a transoral resection of the dens so that the distance from the tumor to the spinal cord was larger. You can see that on this slide.
This is the microscopic rest of the tumor and this is spinal cord. There's now a distance after the surgery, so you can easily make now a CTV-PTV margin of 3 millimeter and treat this patient to a high dose with fractionated IMRT. But, of course, there are other situations where this kind of surgery is not possible and you can make the CTP to PTV margin of 3 millimeters there but you can, obviously, not do that here. This tumor was growing through the intervertebral foramen. And while it made contact with the spinal cord, it was not possible to make a CTV-PTV margin of 3 millimeter. And, well, you have to accept a lower dose in this region of the tumor, of course, if you don't want to overdose the spinal cord.
Concluding. Intrafraction movements are very small in most patients. CTV-PTV margins of 3 millimeters are safe in our view, and when the distance of the CTV to the spinal cord is less than 3 millimeter, either neurosurgery has to be considered or a lower dose in the CTV adjacent to the spinal cord has to be accepted. Thank you.